Process of electrostatic printing



United States Patent 2,735,784 PROCESS OF ELECTROSTATIC PRINTING HaroldG. Greig and Charles J. Young, Princeton, N. J.,

assignors to Radio Corporation of America, a corporation of Delaware NoDrawing. Application July 30, 1953, Serial No. 371,416

Claims. (Cl. 117-175) This invention relates to printing and moreparticularly to novel methods and materials for making permanent printsfrom powder images.

Powder images may be prepared by, several processes, including thoseprocesses which utilize electrostatic means. For example U. S. PatentNo. 1,784,912 to B. F. Scott issued December 16, 19 30, discloses amethod of stenciling on an electrostatically charged surface. By thisprocess, a printing powder is passed through a stencil to a surface thatis electrostatically charged. The powder adheres to the surface therebyforming a powder image where the stencil exposes the surface. U. S.Patent No. 2,297,691 to C. F. Carlson issued October 6, 1942, disclosesmethods of forming electrostatic images by means of photoconductivelayers and U. S. Patent No. 2,357,809 to C. F. Carlson issued September12, 1944, discloses methods of forming electrostatic images by means ofphotoemissive layers. In the latter two processes, a printing powder isdusted on the electrostatic image, and adheres to the electrostaticimage thereby forming a powder image. 1

In many previous processes, some type of pigment must be incorporated inthe printing powder in order to increase the contrast between the powderimage and the background. Specially prepared powders and carefullycontrolled processes are often required. For example, since mostprinting powders have more than one components, it is necessary toproduce an intimate mixture of these components. The components areoften fused together, cooled and then milled to the desired particlesize. If wet milling is used, the mix must be dried and pulverized.Where milling does not reduce the particle size sufliciently, the fusedmaterial is sometimes dissolved in a solvent and then sprayed into achamber that is heated or evacuated or both. The solvent evaporates fromthe spray leaving a fine powder.

An electrostatic image is sometimes formed on a dark colored plate, forexample, a plate coated with photo conductive selenium. A powder imageis formed on this plate and then is transferred to a light coloredprinting base. Since the use of a dark-colored powder on a darkcoloredplate makes it difficult to observe the powder image, a light-coloredprinting powder is more desirable during printing. However, it is oftenpreferred to have a dark image on a light background in the finishedproduct, hence, a printing powder that is light-colored during printingand dark-colored in the final product is desirable, when a dark-coloredimage plate is used.

Many of the pigments previously used in such printing powders tend tofade with time. Hence, it is desirable to employ a pigment which doesnot fade, or one which may be easily reintensified. Another disadvantageof some of the previously employed printing powders is that an adhesiveis often required to fix the powder image to the printing base. Theelimination of such an adhesive provides a more economical and efiicientprocess.

An object of the invention is to provide novel methods and materials forprinting permanent powder images.

p ice Another object of the invention is to provide novel methods andmaterials for printing from electrostatic images.

A'further object of this invention is to eliminate the use of dyes andpigments in improved methods and materials for printing from powderimages.

Another object of the invention is to provide methods and materials forforming a light-colored powder image which may later be converted to adark-colored image print.

A further object of the invention is to provide a powder image 'printwhich may be reintensified easily at some later time.

' An additional object of the invention is to eliminate the necessityfor a separate adhesive to fix a powder image to a'printing base andthus to provide improved methods and materials for producing a fixedvisible image from a powder image.

Another object of the invention is to provide printing powders which areeasily and economically prepared.

In general the foregoing objects may be accomplished in accordance withthe instant invention which includes a method of printing whichcomprises forming a powder image on a printing base, said powder imagecomprising a finely divided and intimate mixture of a zinc-containingmaterial and a resin containing vinyl and chloride radicals, and thenfusing said powder image to said printing base. On fusing, thelight-colored resin and zinc-containing material react to produce a deepblack material, yielding a black image. In one embodiment of theinvention, the powder image may be formed electrostatically and thenfused directly on the printing base. In a second embodie ment, thepowder image may be transferred and fused to a second printing base.This invention also includes a print produced by the method of thisinvention.

The invention also includes a printing powder which comprises afinely-divided and intimate mixture of a resin containing vinyl andchloride radicals and a material containing zinc.

These and other objects will be more apparent and the invention will bemore easily understood from the following detailed description.

Example 1.-50 grams of polyvinyl chloride-acetate resin, for example,composed of polyvinyl chloride and 10% polyvinyl acetate, and 2 grams ofzinc oxide are milled for about 4 hours with quartz pebbles in a smallporcelain ball mill. 0.4 parts of the milled material are mixed withparts of about 0.015 inch diameter glass beads to form the printing mix.

A sheet of paper is coated with a photoconductor in a suitable vehicle.An electrophotographic paper of the type described by H. G. Greig in U.S. patent application No. 248,937 filed September 29, 1951, nowabandoned, provides an excellent photosensitive sheet except that onlythose resins which do not include a resin containing vinyl and chlorideradicals should be used to prepare the coated paper. The surface of thepaper is given a blanket negative electrostatic charge in darkness, forexample, by a corona discharge. The charged surface is exposed to areverse light image. The illuminated areas are thus discharged leavingan electrostatic image on the surface of the paper in the unlightedareas. The above mentioned mixture of beads, zinc oxide and resin iscascaded across the electrostatic image. Particles of the mixture ofresin and zinc oxide adhere to and form a powder image on theelectrostatic image. The paper with the powder image thereon is heateduntil the resin fuses into the paper and reacts with the zinc oxide. Thetemperature range for fusing the resin is about C. to about C. andfusing may be accomplished in several minutes. The resin and the zincoxide are ordinarily white or nearly white in color. Upon fusing withzinc oxide, the resin turns a deep black and adheres to the surface ofthe paper thereby forming a fused black image in a light-coloredbackground. If the print fades upon prolonged exposure to light, it maybe intensified to its original black by reheating to the fusingtemperature of the resin.

Example 2.50 grams of a powdered polyvinyl chloride resin and 2 grams ofzinc oxide are milled for about 4 hours with quartz pebbles in a smallporcelain ball mill. 0.4 part of this material are mixed with 100 partsof about 0.015 inch diameter glass beads to form a printing mix.

A plate, coated with photoconductive selenium, is given a blanketpositive charge, for example, by means of a corona discharge. Thecharged surface is exposed to a positive light image. The illuminatedareas on the surface are discharged leaving an electrostatic image inthe unlighted areas. The above-mentioned printing mix is cascaded overthe electrostatic image. The powder adheres to the electrostatic chargedareas of the plate surface thereby forming a light-colored powder imageon the dark selenium background. The powder image may be inspected muchmore easily at this stage because of the contrast in color between theimage and the background. The powder image is transferred to a sheet ofpaper and then heated until the resin fuses into the paper and reactswith the zinc oxide. The temperature range for fusing the resin is about150 C. to about 175 C. and fusing may be accomplished in less than aminute. The finished print is very similar to the print described inExample 1.

The deep black coloration which occurs when a resin containing vinyl andchloride radicals is fused with a zinccontaining material is animportant feature of the invention. While the reactants initially arelight-covered, when they are fused together, the product is deep blackin color. This coloration is relatively permanent but it may fade to alight brown on prolonged exposure to light. A faded print may berestored to a deep black by reheating the print to the fusingtemperature of the resin.

Any resin containing vinyl and chloride radicals may be used. However,resins chosen from the polyvinyl chlorides, polyvinyl chloridecopolymers such as polyvinyl chloride-acetate, and vinylidene chloridesare preferred.

While zinc oxide and zinc chloride are used in the examples describedherein, any zinc-containing material which does not adversely affect theabove-mentioned reaction may be used. For example, zinc metal, zincoxide, and zinc salts such as zinc sulphate, zinc acetate and zincchloride may be employed. The preferred zinccontaining material is zincoxide.

While paper is a suitable printing base, as in the above examples, othermaterials may be used. The backing is merely a carrier for the finalprint. Hence, the backing may be porous or solid, and may be made of anymaterial which will not interfere with the reaction.

In the above-mentioned examples, the weight of the zinc-containingmaterial is about 4% of the weight of the resin. The proportions ofzinc-containing material to resin is not limited but, preferably, theweight of zinccontaining material should be from 1 to 10% of the weightof resin. The ingredients of the developing powder cannot be blended bythe usual method of fusing together and then grinding the fused productbecause the mixture will turn black. The developing powder of thepresent invention may, however, be prepared by any blending process thatdoes not raise the temperature of the mix above the reactiontemperature. Suitable methods are for example, ball milling as describedabove or spray drying a mixture of zinc-containing material and resin ina solvent.

The powder image may be formed by any convenient method. The foregoingexamples employ an electrostatic image as a means of producing thepowder image. However, the image may be formed by stenciling as in U. S.Patent No. 1,784,912 mentioned heretofore or by other means.Furthermore, the developing powder may be applied to an electrostaticimage by cascading with a mixture of glass beads as described above, orby dusting, or any other convenient means of application.

There has thus been described improved methods and materials forprinting permanent powder images. The uses of pigments of adhesives areeliminated since the reactant materials perform the functions of thesematerials.

What is claimed is:

l. A process of printing which comprises forming a powder image in aparticular configuration on a printing base, said powder imagecomprising a finely-divided and intimate mixture of a zinc-containingmaterial and a resin containing vinyl and chloride radicals, and heatingsaid powder image until said resin and said zinc-containing materialreact to convert said powder image to a black color.

2. A process according to claim 1 wherein said resin is selected fromthe group consisting of polyvinyl chloride, polyvinyl chloridecopolymers and vinylidene chloride, and said zinc-containing material isselected from the group consisting of zinc metal, zinc oxide and zincsalts.

3. A process of electrostatic printing which comprises forming anelectrostatic image in the configuration to be printed on anelectrically-insulating surface, applying a printing powder comprising afinely-divided and intimate mixture of a zinc-containing material and aresin-containing vinyl and chloride radicals to said electrostatic imagethereby forming a powder image and heating said powder image until saidresin reacts with said zinc-containing material to convert said powderimage to a black color.

4. A process according to claim 3 wherein said powder image is fused tosaid electrostatic image-bearing surface.

5. A process according to claim 3 including applying said printingpowder to said electrostatic image, thereby forming a powder image, thentransferring said powder image to a printing base, and then fusing saidpowder image to said printing base.

References Cited in the file of this patent UNITED STATES PATENTS2,067,435 Chatfield et al. Jan. 12, 1937 2,302,361 Yngve Nov. 17, 19422,466,998 Rogers et al Apr. 12, 1949 2,618,551 Walkup Nov. 18, 1952OTHER REFERENCES Vinylite Resins for Surface Coatings, publication ofthe Bakelite Corp, Copyright 1942; pages 15, 20 and 21 relied upon.

1. A PROCESS OF PRINTING WHICH COMPRISES FORMING A POWDER IMAGE IN APARTICULAR CONFIGURATION ON A PRINTING BASE, SAID POWDER IMAGECOMPRISING A FINELY-DIVIDED AND INTIMATE MIXTURE OF A ZINC-CONTAININGMATERIAL AND A RESIN CONTAINING VINYL AND CHLORIDE RADICALS, AND HEATINGSAID POWDER IMAGE UNTIL SAID RESIN AND SAID ZINC-CONTAINING MATERIALREACT TO CONVERT SAID POWDER IMAGE TO A BLACK COLOR.